The present invention relates to organic optical devices such as waveguides, or more particularly to lithographically formed optical waveguides employing polymeric materials having low optical loss, good long term and short term stability, good flexibility and reduced stress or crack induced optical scattering loss.
In optical communication systems, messages are transmitted by carrier waves at optical frequencies that are generated by such sources as lasers and light-emitting diodes. There is interest in such optical communication systems because they offer several advantages over conventional communication systems. They have a greatly increased number of channels of communication as well as the ability to transmit messages at much higher speeds than electronic systems using copper wires. This invention can be used in the formation of the light transmissive element of optical devices. Illustrative of such devices are planar optical slab waveguides, channel optical waveguides, rib waveguides, optical couplers, optical splitters, optical switches, micro-optical elements and the like which are described in more detail in U.S. Pat. Nos. 4,609,252; 4,877,717; 5,136,682; 5,481,385; 5,462,700; 5,396,350 and 5,428,468. All of the foregoing patents are incorporated herein by reference. One preferred means for switching or guiding waves of optical frequencies from one point to another is by an optical waveguide. The operation of an optical waveguide is based on the fact that when a medium which is transparent to light is surrounded or otherwise bounded by another medium having a lower refractive index, light introduced along the inner medium""s axis is highly reflected at the boundary with the surrounding medium, thus producing a guiding effect.
It is known in the art to produce polymeric optical waveguides and other optical interconnect devices which transport optical signals in optical circuitry or fiber optic networks. One method used to form an optical device involves the application of standard photolithographic processes. Photopolymers are of particular interest for optical interconnect applications because they can be patterned with photolithographic techniques which have been well developed. By this process, lithographic processes are used to define a pattern in a light sensitive layer deposited on a chosen substrate. By standard photolithographic processes, a pattern is developed in the light-sensitive material. Among the many known photopolymers, acrylate materials have been widely studied as waveguide materials because of their optical clarity, low birefringence and ready availability of a wide range monomers. However, the optical performance of such a structure has been poor, since optical losses as well as the aging resistance of exposed material have been unsatisfactory. The devices made from these materials have high optical loss and have turned from clear to yellow and then to brown during thermal baking at high temperatures (xe2x89xa7120xc2x0 C.) in air, exhibiting increased absorption loss. Thermal stability of waveguide materials is extremely important for practical device applications. Thermal degradation of materials during prolonged use at high temperatures will cause failure of a device. Additionally, for optical polymers, thermal yellowing will increase optical loss and will reduce the lifetime of the device. The other problem associated with the earlier generation acrylic materials is that they are very brittle and the devices made from them crack upon baking. Waveguides often need to be bent 90 degrees either in-plane or out-of-plane. This requires flexible cladding and core materials to avoid cracking and optical losses due to bending-induced stress. An object of the invention is to provide materials with low optical loss, good thermal stability, good adhesion to a variety of substrates and sufficient flexibility to allow right angle bends.
U.S. Pat. No. 4,609,252 teaches one method of lithographically forming optical elements using an acrylic photoactive composition which is capable of forming a waveguide material upon polymerization. However, this patent teaches one to utilize polymers with as high a glass transition temperature as possible in order to provide for the greatest operating temperatures. Glass transition temperatures of 90xc2x0 C.-220xc2x0 C. are required. U.S. Pat. No. 5,136,682 teaches the production of waveguides using light polymerizable compositions such as acrylics having a Tg of at least 100xc2x0 C. The foregoing waveguides suffer from undesirably high optical loss and are not very flexible.
It would be desirable to produce optical devices from polymeric materials which have short term thermal stability at temperatures above 200xc2x0 C. for device integration and packaging, long term stability at 120xc2x0 C. for long term operation, low absorption and scattering loss at application wavelengths, flexibility to facilitate device designs in different geometry, have precisely controllable refractive indexes for mode and numeric aperture control, and compatibility with existing technologies.
The present invention provides polymers for optical devices that address the above-mentioned problems. The polymeric materials offer low optical loss (0.02 dB/cm at a wavelength of 810 nm), good long term stability at 120xc2x0 C. and short term stability at 200xc2x0 C. Aging induced loss is lower than 0.1 dB/cm over a device lifetime of five years or longer at 125xc2x0 C. Good flexibility enables the materials to be used in various device geometries and reduces stress or cracking induced scattering loss. Precise refractive index control allows control of mode and numeric aperture and permits fabrication of single mode waveguides that match single-mode fibers in both cross sectional dimensions and numeric aperture. The compositions also have sufficient adhesion to a variety inorganic and organic substrates. The materials are colorless and exhibit extremely low intrinsic absorption loss between 400 and 1000 nm. Their flexibility minimizes scattering losses caused by stress and microcracks. Their high photolithography contrast allows for formation of smooth walls and thus reduces scattering loss at the interfaces of a multi-layer structure. All of these material properties contribute to the extremely low losses of the waveguides. Thermally induced optical losses due to yellowing are minimized. These core and cladding materials are miscible with one another, so the index at each layer of a waveguide can be precisely tailored by mixing selected pair of high index and low index solutions. This property can be used to precisely control the mode of a waveguide and can be used to fabricate large-size single-mode waveguides that match commercial single-mode fibers in both cross sectional dimensions and numeric aperture.